Grinding machine

ABSTRACT

A grinding machine of the controlled-force type having a hydraulic system permitting change from low force to high force grinding in which an air trap is provided in the hydraulic system to cause the force to change relatively slowly from low force to high force.

United States Patent 1191 Kuniholm 1 GRINDING MACHINE [75] Inventor: Martin L. Kuniholm, Worcester,

Mass.

[73] Assignee: Cincinnati Milacron-Heald Corporation, Worcester, Mass.

[22} Filed: Nov. 19, 1973 [21] Appl. No.: 416,980

1 51 Aug. 12, 1975 3,714,741 2/1973 Uhtenwoldt 51/l65.91

Primary Examiner l-larold D. Whitehead Attorney, Agent, or FirmNorman S. Blodgett; Gerry A. Blodgett [5 7] ABSTRACT 6 Claims, 4 Drawing Figures [52] US. Cl 51/165.9; 60/413 51 1m.c1. B24b 49/08 [58] Field of Search 51/165 R, 165.77, 165.9, 51/165.91, 165.92; 60/413 [56] References Cited UNITED STATES PATENTS 2,554,381 5/1951 Patterson 60/413 X Z- "Z LJ IM 4I fFEED 34 I 23 SI PATENTED AUG 1 2 [975 SHEET GRINDING MACHINE BACKGROUND OF THE INVENTION In the operation of grinding machines and particularly those of the controlled-force type, it is common practice to provide for two levels of grinding force. At

feed the abrasive wheel toward the workpiece surface and then to switch to the higher force level when contact is made between the wheel and the workpiece. Since the feeding usually takes place by means of a hydraulic cylinder, it is only necessary to use a simple solenoid switch to detach the low pressure fluid source and to attach the high pressure fluid source to the cylinder. The changeover is quite rapid and presents no problem in many kinds of grinding applications. There are, however, many situations where the initial engagement of the abrasive wheel with the workpiece takes place over a limited area and this means that, no matter when the switchover from low force to high force takes place, the high force changeover exists at such a time that only limited contact is taking place between the wheel and the workpiece. This means that the wheel will be subjected to extremely high unit stress and this results not only in a shortened life for the wheel, but also, where the shape of the wheel is critical, as in plunge grinding, the shape of the finished surface may be other than the shape that was introduced into the wheel by dressing. This is particularly true in the plunge grinding of bearing races where the radius of curvature of the unfinished groove is less than the radius of the abrasive wheel (which is dressed to the radius of the finished workpiece surface of the groove). Engagement of a large radius wheel with a small radius groove with considerable force causes the corners of the groove to cut grooves in the surface of the wheel and destroy its geometric form and shape. These and other difficulties experienced in the prior art devices have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide a grinding machine for plunge grinding of noncylindrical surfaces in which the feeding force between the wheel and the workpiece is maintained at a low value until substantial area of contact between the wheel and the workpiece is arrived at.

Another object of this invention is the provision of a grinding machine in which the life of the abrasive wheel is substantially long despite the fact that the first contact between the wheel and the workpiece may be of limited area.

A further object of the present invention is the provision of a grinding machine in which the shape of the abrasive wheel remains substantially as dressed irrespective of the fact that engagement between the wheel and the workpiece may take place in a limited area.

It is another object of the instant invention to provide a grinding machine for particular use, where the initial contact between the wheel and the workpiece may take place in a limited area of the wheel, but the grinding force is maintained at a value such that the unit stress on the wheel is maintained below a value that would cause damage to the wheel.

A still further object of the invention is the provision of a grinding machine having two levels of force in which the transition between the low level and the high level takes place with the increase at a rate which will maintain the unit pressure between the wheel and the workpiece at substantially the same value during the transition.

It is a further object of the invention to provide a grinding machine capable of grinding at high force in which the wheel is introduced into the work in such a way that the unit stress remains substantially the same during the introduction.-

It is a still further object of the present invention to provide a grinding machine in which the abrasive wheel is introduced into the work with increasing force and the increase in force takes place in straight line relationship.

With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.

SUMMARY OF THE INVENTION In general, the invention consists of a grinding machine having a controlled force feed mechanism including an actuator, having a selector valve connected by a conduit to the actuator, having a source of lowpressure fluid connected to the valve, and having a source of high-pressure fluid connected to the valve.

The selector valve is operative to connect either source to the conduit and to the actuator. An air trap communicates with the conduit to control the rate of build-up of fluid pressure in the actuator when the selector valve connects the source of high pressure fluid to the actuator. An adjustable restriction is provided to control the time rate of force build-up.

BRIEF DESCRIPTION OF THE DRAWINGS The character of the invention, however, may be best understood by referenceto one of its structural forms, as illustrated by the accompanying drawings, in which:

FIG. 1 is a perspective view of a grinding machine embodying the principles of the present invention,

FIG. 2 is a diagram showing certain relationships between an abrasive wheel and a workpiece during grinding,

FIG. 3 is a graph showing the relationship between force and time both in the case of prior art devices and in the case of the present invention, and

FIG. 4 is a schematic diagram of hydraulic circuitry forming part of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, wherein are best shown the general features of the invention, the grinding machine indicated generally by the reference numeral 10, is of the type shown and described in the patent of Uhtenwoldt et al US. Pat. No. 3,570,185 which issued on Mar. 16, 1971. It is provided with a lower base 11 on which is mounted a workhead 12 and a wheelhead 13. Around the front of the base extends a splash guard 14 which is readily removable. Extending upwardly from the rear of the base 11 is a super-structure 15 having two arms, similar to the arm 16, which extend forwardly from the ends of the base. Mounted between the arms is a control cabinet 17. At one end of the machine is located a coolant tank 18 receiving coolant returned from the machine through a pipe 20.

FIG. 2 shows the type of grinding problem solved by the present invention. An abrasive wheel 19 is mounted on a spindle in the wheelhead 13, while a workpiece 21 is rotatably carried in the usual way in the workhead 12. The workhead is shown as the outer race of a ball bearing that is to be provided with a groove 22 which is generally semi-circular in cross-section. For that purpose, the periphery of the wheel 19 is provided (by means of a rotary dressing arrangement) with a peripheral surface 23 which is of exactly the same shape as the surface of the groove 22 to be ultimately formed. However, the workpiece 21 (before the grinding operation) has been provided with a rough groove 24 which is generally concentric with the desired finished surface 22, but has, of course, a smaller radius of curvature. This means that, when contact first takes place between the surface 23 of the wheel 19 and the surface 24 of the workpiece 21, engagement takes place at two relatively sharp corners 25 and 26 of the workpiece. In conventional grinding practice, the force between the wheel and the workpiece is increased to a high value either at or slightly after the moment of contact. This produces a very high unit stress on the wheel and, of course, on the workpiece at the corners 25 and 26. The net result is that the surface 23 of the wheel is deformed. As a practical matter, it is provided with a notch on its periphery which notch will be duplicated in the finished surface 22 in the normal course of events. It is this type of problem that it is the purpose of the present invention to avoid.

FIG. 3 shows a graphical representation of the problem. The graph shows the relationship between force and time in the introduction of an abrasive wheel into a workpiece in the manner described above. Assuming that the wheel is brought in with a force at level A and that grinding is to take place at a higher force level B, the prior art transition line brings about a contact point between the wheel and the workpiece at a very high force level even though the force has not reached the level B as yet. It is the purpose of the present invention to cause the contact to take place at a lower level, perhaps only slightly larger than the lower level A. As shown in the drawing, the prior art arrangement results in an initial contact force at a high force C, while the present invention brings about an initial contact force at a low force D.

FIG. 4 shows the controlled-force feed arrangement provided in the grinding machine in accordance with the present invention. The grinding machine has a hydraulic controlled-force feed mechanism, indicated generally by the reference numeral 27, which is similar to the feed mechanism shown and described in the above-mentioned patent of Uhtenwoldt et al U.S. Pat. No. 3,570,185. Furthermore, the mechanism is associated with controls of the type shown in the aforementioned patent for regulating the grinding cycle. The feed mechanism is provided with a hydraulic cylinder 28 connected by a conduit 31 to a solenoid-type selector valve 32. This valve has one outlet which is connected to the conduit 31 and it has two inlets, one of which is connected by a conduit 33 to a source of highpressure hydraulic fluid, while the other inlet is connected by a conduit 34 to a source of low-pressure hydraulic fluid. The conduit 33 is connected through a check valve 35 and a variable restrictor 36 to a highpressure regulator valve 37 whose inlet is connected to a high-pressure fluid line 38.' The other conduit 34 is connected through a check valve 49 to the output of a low-pressure regulator valve 39 whose inlet is also connected to the pressure line 38. This portion of the feed mechanism 27 operates in the conventional way in providing a low force to the feed cylinder 28 and at a certain time in the grinding cycle shifts to a high-force oil pressure to the feed cylinder 28.

The conduit 31 (which is attached to the output of the selector valve 32) is also connected to the bottom end of an air trap 41. This air trap consists of a primary tube 42 which is closed at its top and the bottom ends. Extending into the top end is a secondary tube 43 which extends concentrically through the tube 42 to a point somewhat above the bottom of the tube 42 at the level X-X. The secondary tube 43 extends Well above the tube 42 and is connected to a gage 44. It is also connected by a conduit 45 to a normally-open solenoid valve 46, the other side of which is connected to an upwardly-extending branch 47 whose outer end is bent over to prevent the admission of dust and which is exposed to the atmosphere and also connected to a downwardly-directed branch 48 leading to an oil sump.

The operation of the apparatus will now readily be understood in view of the above description. The grinding cycle of the machine is controlled by hydraulic solenoid valves which, in turn, are regulated by electrical connections operating on the coils of the solenoids. The electrical control apparatus is of the conventional type and is well known in this art. At a certain time in the cycle the wheel 19 is advanced laterally toward the workpiece 21. Both the wheel 19 and the workpiece 21 are rotated by the wheelhead l3 and the workhead 12,

tion. The advancement of the wheel laterally takes place under the impetus of the cylinder 28 operating by means of low-pressure fluid in the conduit 34. This is the condition of the apparatus shown in FIG. 4 when the low pressure passes through the selector valve 32 into the conduit 31. This is the force situation indicated in FIG. 3 as the force level A. As the abrasive wheel is about to contact the workpiece, the selector valve 32 is actuated at the damper-in position in FIG. 3, thus connecting the conduit 33 to the conduit 31 and allowing high-pressure fluid from the regulator valve 37 to flow toward the cylinder 28. Theoretically, the switching by means of the selector valve 32 would be expected to cause high-pressure fluid to instantaneously appear in the cylinder 28, but, as a practical matter, all of the conduits, valves, and parts of the apparatus are somewhat flexible (even though the fluid itself is only slightly compressible) and a certain amount of energy is absorbed by these resilient elements so that the force passes from the force A (in FIG. 3) to the force B in the cylinder along a fairly steep slope indicated in FIG. 3 as PRIOR ART. Soon after the'selector valve 32 operates, the contact point is reached between the wheel and the workpiece, so that the condition shown in FIG. 2 arises. At that time the force behind the wheel is at the force level C of FIG. 3 and is a high force. This causes the damage to the wheel 19 that was described above, were it not for other parts of the apparatus. In the present invention, when the selector valve 32 is operated, the high-pressure fluid flows into the conduit 31. It also appears at the bottom of the air trap 41. Since the machine has been started and the timer has timed out, the solenoid valve 46 has closed. Furthermore, the fluid is at some level Y-Y due to low force also makes itself felt in the air trap 41. As a result of the spring-like action of the compressed air in the top of the tube 42, the force which appears in the cylinder 28 moves along the line in FIG. 3 that is indicated as IN- VENTION which has a much lower slope. In other words, it takes longer for the pressure to rise in lines 27 and 31 from low level 39 to high level 37. The grinding force between the wheel and the workpiece rises in proportion to the rate of pressure increase. This means that, when the contact takes place between the wheel and the workpiece, it takes place at a very low force level, indicated as level D in FIG. 3. This prevents damage from taking place because of contact between the corners 25 and 26 of the workpiece and the surface 23 of the wheel. As a matter of fact, it will be understood, by examining F IG. 2, that, as the grinding proceeds, the corners 25 and 26 will be flattened out and approach the shape of the surface 23 of the wheel. This means that a still larger area of contact exists between the wheel and the workpiece, but, since the force during this time is increasing along the relatively flat slope in FIG. 3, the increase in area of contact increases at approximately the same rate as the force increases, so that the unit stress is maintained at a more or less constant value, certainly at a value below the unit stress value in which the surface of the wheel will be damaged. By proper adjustment of the variable restrictor 36, it is possible to select the slope of the INVEN- TION" curve in FIG. 3 in such a way that the unit stress stays the same, in the sense that the rate of increase in contact area between the wheel and the workpiece is practically the same as the rate of increase of the force.

One problem that exists in an apparatus of this kind is that the hydraulic fluid has a tendency to absorb air as time passes and provision is made in the present apparatus to introduce more air while the machine is shut down. As has been noted, the valve 46 remains closed so long as the grinding machine is operative. This is accomplished simply by having the coil of the solenoid valve in series with a main switch of the grinding machine and a timer. When the machine is shut down, the valve automatically opens, thus connecting the conduit 45 to the branches 47 and 48. When the machine power is shut off, valve 46 opens and air pressure trapped in the tube 42 forces fluid to flow up the tube 43 and down through the branch 48 into the sump. Oil also exhsusts through low elevation port 40 as air enters the system through the branch 47 to fill the accumulator. When the machine is started up again, valve 46 remains open for a short length of time as determined by a timer formed in the machine. In this way the lowforce pressure liquid enters the bottom of the tube 42 and pushes the air out through the valve 46 until the fluid reaches the level XX. As the fluid continues to rise and even enters the tube 43, the volume of 42 (less the volume of tube 43) of air at atmospheric pressure is trapped in the top of the tube 42. The timer times out, the valve 46 closes, and the fluid rises to an intermediate level, such as YY which, of course, is determined by the low-pressure regulator valve 39 setting. The feed system is now ready for grinding. Thus, the air volume is re-established each time the machine is shut down to compensate for air absorbed by the pressurized fluid.

It can be seen than that many adantages arise because of the above construction. Because the unit pressure is maintained below a destructive value, the surface of the wheel will remain true despite plungegrinding of irregular surfaces.

It is obvious that minor changes may be made in the form and construction of the invention without departing from the material spirit thereof. It is not, however, desired to confine the invention to the exact form herein shown and described, but it is desired to include all such as properly come within the scope claimed.

The invention having been thus described, what is claimed as new and desired to secure by Letters Patent 1. A grinding machine, comprising:

a. a base,

b. a workhead tabl'e mounted on the base and adapted to carry a workpiece,

c. a wheelhead table mounted on the base and adapted to carry a wheelhead in which is mounted a spindle carrying an abrasive wheel,

d. a controlled-force feed mechanism connecting the base to one of the tables to produce relative movement between the tables to cause engagement of the abrasive wheel and the workpiece for a grinding operation, the feed mechanism including an actuator,

e. a selector valve connected by a conduit to the actuator,

f. a source of low-pressure fluid connected to the valve,

g. a source of high-pressure fluid connected to the valve, the selector valve being operative to connect either source to the conduit and to the actuator, and

h. an air trap communicating with the conduit to retard the build-up of fluid pressure in the actuator when the selector valve connects the source of high-pressure fluid to the actuator, the air trap consisting of a vertical primary tube closed at the ends, a conduit leading from the bottom of the tube to the actuator, and a secondary tube entering the top of the primary tube and extending coaxially from the top end to a position spaced from the bottom, the secondary tube extending from the top of the primary tube, being connected to one side of a solenoid valve which opens when the grinding machine is stopped, and the other side of the solenoid valve being connected to a fitting which has an upwardly-extending branch leading to the atmosphere and a downwardly-extending branch leading to oil storage.

2. A grinding machine as recited in claim 1, wherein the actuator is a hydraulic cylinder, and wherein the source of high-pressure fluid includes a pressure regulator and a flow-regulating orifice.

3. A grinding machine as recited in claim 1, wherein a pressure-indicating gage is connected to the secondary tube.

4. A grinding machine as recited in claim 1, wherein means is provided for restoring to the trap quantities of air lost from time to time and wherein the said means is operative only when the grinding machine is shut off.

5. A grinding machine as recited in claim 4, wherein shutting off the grinding machine opens a solenoid valve which serves to bleed off a quantity of hydraulic fluid and introduce a quantity of air.

6. A grinding machine as recited in claim 5, wherein a timer is provided to keep the solenoid valve open for a period of time after the machine is started up. 

1. A grinding machine, comprising: a. a base, b. a workhead table mounted on the base and adapted to carry a workpiece, c. a wheelhead table mounted on the base and adapted to carry a wheelhead in which is mounted a spindle carrying an abrasive wheel, d. a controlled-force feed mechanism connecting the base to one of the tables to produce relative movement between the tables to cause engagement of the abrasive wheel and the workpiece for a grinding operation, the feed mechanism including an actuator, e. a selector valve connected by a conduit to the actuator, f. a source of low-pressure fluid connected to the valve, g. a source of high-pressure fluid connected to the valve, the selector valve being operative to connect either source to the conduit and to the actuator, and h. an air trap communicating with the conduit to retard the build-up of fluid pressure in the actuator when the selector valve connects the source of high-pressure fluid to the actuatoR, the air trap consisting of a vertical primary tube closed at the ends, a conduit leading from the bottom of the tube to the actuator, and a secondary tube entering the top of the primary tube and extending coaxially from the top end to a position spaced from the bottom, the secondary tube extending from the top of the primary tube, being connected to one side of a solenoid valve which opens when the grinding machine is stopped, and the other side of the solenoid valve being connected to a fitting which has an upwardly-extending branch leading to the atmosphere and a downwardly-extending branch leading to oil storage.
 2. A grinding machine as recited in claim 1, wherein the actuator is a hydraulic cylinder, and wherein the source of high-pressure fluid includes a pressure regulator and a flow-regulating orifice.
 3. A grinding machine as recited in claim 1, wherein a pressure-indicating gage is connected to the secondary tube.
 4. A grinding machine as recited in claim 1, wherein means is provided for restoring to the trap quantities of air lost from time to time and wherein the said means is operative only when the grinding machine is shut off.
 5. A grinding machine as recited in claim 4, wherein shutting off the grinding machine opens a solenoid valve which serves to bleed off a quantity of hydraulic fluid and introduce a quantity of air.
 6. A grinding machine as recited in claim 5, wherein a timer is provided to keep the solenoid valve open for a period of time after the machine is started up. 